1. Field of Invention
This invention relates to liquid toner development of latent electrostatic images. More particularly, this invention relates to liquid toner development systems and methods that are capable of modifying image data to compensate for detected impurities in the toners.
2. Description of Related Art
Digital color printing devices, such as ink jet printers, ionographic printers, laser printers, copiers and the like, receive image data, which may be internally or externally generated, in the form of signals to print specified colors in specified areas. Process color printers print all specified colors as some combination of halftone patterns of the four process colors, cyan, magenta, yellow and black, conventionally labeled C, M, Y and K. Many kinds of digital printer control systems are known. The input color can be specified as a combination of red, green and blue values (R, G, B values) such as are used in computer monitor displays, by a unique identification number (such as a number from the Pantone.RTM. Color Matching System), by color space coordinates (such as CIELAB's L* a* b* coordinates), or by other color specification systems. The specification of the input color can also be provided as a set of percent area coverages for the four process color. The digital printer control system converts the input color to an on-off pattern for each of the process colors. The digital printer control system can use look-up tables or formulas or multi-step algorithms to determine the halftone process colors patterns that best reproduce each input color. These halftone patterns can take the form of lines and spaces between the lines or, more commonly, dots and spaces between the dots. When the on-off patterns specify dots and spaces between dots, the dots can be round, oval, or even polygonal. The pattern of dots can be regular or random. The same methods used for converting color areas to process color halftone patterns may also be used in offset, gravure, letterpress and other printing systems to produce printing plates, printing cylinders and the like for the process colors. The digital printer control system finally converts the on-off process color patterns into on-off signals for a device which will construct the process color patterns. In many digital printing systems a single halftone dot is produced from a set of smaller dots.
A typical electrostatographic printing machine employs a photoconductive member that is sensitized by charging the photoconductive member to a substantially uniform potential. The charged portion of the photoconductive member is image-wise discharged by light to form a latent image of an original image on the photoconductive member. Exposing the charged photoconductive member with light selectively dissipates the charge to form the latent image on the charged photoconductive member. The latent image recorded on the photoconductive member is developed using a developer material. The developer material can be a liquid developer material known in the literature as "liquid electrophoretic ink" or simply "liquid ink" or "liquid xerographic toner" or simply "liquid toner". In a liquid development system, the photoconductive surface is contacted by liquid developer material comprising finely divided toner particles dispersed in an insulating liquid carrier. The latent image attracts the toner particles dispersed throughout the insulating liquid carrier material particles to the photoconductive surface to develop the latent image, thus forming a visible image.
Liquid toners have many advantages and often produce images of higher quality than images formed with powder toners. For example, images developed with liquid toner may adhere to the copy substrate without requiring fixing or fusing to the copy substrate. Thus, the liquid toner may not need to include a resin for fusing purposes. In addition, the toner particles suspended in the liquid carrier material can be made significantly smaller than the toner particles used in powder toners. Using such small toner particles is particularly advantageous in multicolor processes where multiple layers of toner particles generate the final multicolor output image. An additional advantage of liquid toners is that the particles are charged by a controlled chemical reaction between the sites on the particle surface and molecules dissolved in the liquid carrier material. This charging makes possible liquid toner particles with 20-50% pigment, instead of the 2-10% pigment which is common in dry toner particles. This increased pigment loading reduces the amount of resin contained in the image transferred to the final printed substrate. This reduced resin reduces paper curl and leads to multicolor output images which generally have a significantly more uniform finish compared to images formed using powder toners.
Liquid toners typically contain about 1-5% by weight of fine solid particulate toner material disbursed in the liquid carrier material. The liquid carrier material is typically a hydrocarbon. After developing the latent electrostatic image, the developed image on the photoreceptor may contain 6-25% by weight of the solid particulate toner particles along with residual liquid hydrocarbon carrier. To complete the development process, the solid particulate toner material is typically compacted onto the photoreceptor and the excess liquid carrier material removed from the photoreceptor.
Liquid toner development systems are generally capable of very high image resolution because the toner particles can safely be ten or more times smaller than dry toner particles. Typical dry toner particles are on the order of 10 microns in diameter. Typical liquid toner particles are on the order of 1 micron in diameter. Liquid toner development systems show impressive grey scale image density response to variations in image charge and achieve high levels of image density using small amounts of liquid developer. Additionally, the systems are usually inexpensive to manufacture and are very reliable.